A fluorinated copolymer is excellent in voltage resistance, oxidation resistance and chemical resistance. Accordingly, use as a binder in a storage battery device such as a capacitor, a primary battery or a secondary battery for an electronic device or an electric car, for which a high output power, a high capacity and excellent cycle characteristics are required, has been proposed.
For example, Patent Document 1 discloses an example wherein a secondary battery was prepared by using, as a binder, a fluorinated copolymer which is a copolymer of tetrafluoroethylene and propylene, wherein the molar ratio of structural units derived from tetrafluoroethylene/structural units derived from propylene was 56/44.
Further, Patent Document 2 proposes a binder containing a water-soluble polymer material (preferably water-soluble cellulose) which will not swell in an electrolytic solution and a fluororesin (such as polytetrafluoroethylene or tetrafluoroethylene/hexafluoropropylene) to impart flexibility to the positive electrode.
The binder as disclosed in Patent Document 2 is less likely to swell or be dissolved in an electrolytic solution, and a decrease in the bonding property is suppressed, and accordingly it is considered that a lithium ion secondary battery having favorable cycle characteristic and high temperature property is obtained with such a binder.
However, the binder used in Examples in Patent Document 1 is excellent in the adhesion but is not insufficient in view of suppression of swelling in an electrolytic solution (electrolytic solution resistance).
Further, with respect to the binder as disclosed in Patent Document 2, since the adhesion of the fluororesin contained in the binder is low, the electrode active material fixed to the electrode is likely to fall down.